I have a question! Mars' moon...

Can anyone answer?

Does Mars' moon always point towards the planet?

(like our moon always has the same face to Earth, because each body
has equal and opposite rotation "due to tidal forces")

also, if I can have another, is there any hard data on these tidal
forces, and how long it took for them to create the equal&opposite
rotation effect?

cheers

D

Diesel wrote:

Does Mars' moon always point towards the planet?

(like our moon always has the same face to Earth, because each body
has equal and opposite rotation "due to tidal forces")

Both of them do, yes. And both of them have their long axes
pointing towards Mars' center. (Phobos and Deimos are notably
aspherical.)

And it's not so much that Earth and the Moon have "equal and
opposite rotation". Tidal effects tend to pull on irregularities in such
a way as to cause most of the mass to be concentrated in the direction of
the body pulling on it. (The Moon's center of mass is slightly displaced
from its geographical center -- and that center of mass points Earthward
as seen from the geographical center.) The effect becomes more pronounced
with bodies close to one another in space or in mass.

also, if I can have another, is there any hard data on these tidal
forces, and how long it took for them to create the equal&opposite
rotation effect?

Eep. The most I've ever seen on this subject is that it takes
billions of years, but exactly how long? I don't know. It is significant
to note that the vast majority of moons in the Solar System are tidally
locked to their primaries, though (and Pluto and Charon are even tidally
locked to one another).
--
-- With Best Regards,
Matthew Funke )

"Matthew F Funke" schrieb im Newsbeitrag
...
Diesel wrote:
Eep. The most I've ever seen on this subject is that it takes
billions of years, but exactly how long? I don't know. It is significant
to note that the vast majority of moons in the Solar System are tidally
locked to their primaries, though (and Pluto and Charon are even tidally
locked to one another).
But since one could view planets as the suns "moons", why are they not
tide locked with the sun?

Lots of Greetings!
Volker

Matthew F Funke replied to Volker Hetzer:

why are [planets] not tide locked with the sun?

[I disliked the wording of Volker's question so much that I
felt compelled to modify it. --jr]

Well, Mercury is locked in a sort of resonance with the Sun --
kind of a simple tidal locking. As far as the rest is concerned,
remember that tidal locking becomes much more pronounced as the
bodies in question get closer to each other (tides go down with
the *cube* of distance, not just the *square* as gravity does) and
as they get close to each other in mass. The planets are *much*
smaller than the Sun.

Similarity of mass can't be a factor. Gravity-gradient
stabilization has been used on many low Earth orbit satellites,
such as some of the US Navy Transit satellites in the 1960s,
the GEOS satellites in the 60s and 70s, and Geosat in 1985.

Consider the Sun and the Earth versus the Earth and the Moon.
The Sun is about 330,000 times as massive as the Earth, but the
Earth is only about 81 times as massive as the Moon. The Earth and
the Moon are *much* closer to each other in terms of mass than the
Sun is to the Earth, which contributes to the Moon's faster tidal
locking. (Note that this point cannot be taken strictly at face
value, since proximity in mass is only one of the primary
contributors to tidal locking... but it might give you some
insight as to what's going on.)

Mars is almost 60 million times the mass of Phobos, and more
than 350 million times the mass of Deimos.

I suspect that the idea is one you came up with, yourself.
Not finding any information contradicting it, you gradually
assumed that it was correct, because it seemed to fit the
info you had. Yes? No? Maybe?

-- Jeff, in Minneapolis

..

Jeff Root wrote:
Matthew F Funke replied to Volker Hetzer:

Well, Mercury is locked in a sort of resonance with the Sun --
kind of a simple tidal locking. As far as the rest is concerned,
remember that tidal locking becomes much more pronounced as the
bodies in question get closer to each other (tides go down with
the *cube* of distance, not just the *square* as gravity does) and
as they get close to each other in mass. The planets are *much*
smaller than the Sun.

Similarity of mass can't be a factor. Gravity-gradient
stabilization has been used on many low Earth orbit satellites,
such as some of the US Navy Transit satellites in the 1960s,
the GEOS satellites in the 60s and 70s, and Geosat in 1985.

As I'm sitting here thinking about it, your statement makes sense to
me. After all, tide locking has more to do with tidal torque than
anything, and the similarity of mass of the bodies doesn't seem to factor
into that at all. What matters is the force of the gravity and the drag
(or pull) that force creates on tides raised.

This seems borne out by the Earth-Moon system. Gravity is a
symmetrical force, after all, but the Moon is much smaller, so the Earth
would be more effective at tidally locking the Moon.

Consider the Sun and the Earth versus the Earth and the Moon.
The Sun is about 330,000 times as massive as the Earth, but the
Earth is only about 81 times as massive as the Moon. The Earth and
the Moon are *much* closer to each other in terms of mass than the
Sun is to the Earth, which contributes to the Moon's faster tidal
locking. (Note that this point cannot be taken strictly at face
value, since proximity in mass is only one of the primary
contributors to tidal locking... but it might give you some
insight as to what's going on.)

Mars is almost 60 million times the mass of Phobos, and more
than 350 million times the mass of Deimos.

I suspect that the idea is one you came up with, yourself.
Not finding any information contradicting it, you gradually
assumed that it was correct, because it seemed to fit the
info you had. Yes? No? Maybe?

I doubt I came up with it myself, since it seems so bizarre. But I
can't for the life of me nail down where I first heard it. Perhaps I had
misheard something about the similarity in mass of the Earth-Moon system
way back when in the context of tidal locking or something, and somewhere
my brain assumed it was relevant, not really taking the time to think out
why it might or might not be so.

Besides, it's not like me to invent tidal theories as I go... a
casual look at trying to figure out how the Moon's tidal influence relates
to oceanic tides convinced me a long time ago that there was more to this
than met the eye.

In any case, thanks for forcing me to re-examine things and bringing
me to task. I apologize for not thinking this through.

I've re-examined (and, more importantly, researched) the statements I
made about tides, and still stick by distance as an important factor
(since tide strength is inversely proportional to the cube of distance).
Thus, the effect of the Moon on tides is approximately

[(7.349e22 kg)/(384403 km)^3]/[(1.989e30 kg)/(149597890 km)^3]

= 2.18 times the effect of the Sun's effect on tides.

(Note that even though I was fast and loose with units, they cancel
out.) Thus, one can say that the Sun hasn't tidally locked the planets
because the interplanetary distances are much greater than the distances
between the planet and its moon(s). Mass is important, yes, but distance
is much *more* important.

And closeness of mass of the two bodies doesn't have squat to do with
anything, and I apologize for parroting that.
--
-- With Best Regards,
Matthew Funke )

In message , Jeff Root
writes
Matthew F Funke replied to Volker Hetzer:

why are [planets] not tide locked with the sun?

[I disliked the wording of Volker's question so much that I
felt compelled to modify it. --jr]

Well, Mercury is locked in a sort of resonance with the Sun --
kind of a simple tidal locking. As far as the rest is concerned,
remember that tidal locking becomes much more pronounced as the
bodies in question get closer to each other (tides go down with
the *cube* of distance, not just the *square* as gravity does) and
as they get close to each other in mass. The planets are *much*
smaller than the Sun.

Similarity of mass can't be a factor. Gravity-gradient
stabilization has been used on many low Earth orbit satellites,
such as some of the US Navy Transit satellites in the 1960s,
the GEOS satellites in the 60s and 70s, and Geosat in 1985.

Isn't the important factor the gradient across the object? As you say,
it's actually called gravity-gradient stabilisation when it's applied to
artificial satellites (it's also more syllables than "tidal locking", so
it sounds cool).
--
"Forty millions of miles it was from us, more than forty millions of miles of
void"